1. Field of the Invention
The present invention relates to a charge and discharge control circuit and an apparatus for preventing overcharge and overdischarge in a secondary battery pack such as a lithium-ion secondary battery used in various types of portable equipment. The present invention particularly relates to a charge and discharge control circuit and an apparatus which are optimum in view of battery life and ensuring the safety of a battery.
2. Prior Art
Secondary batteries such as a lithium-ion secondary battery are frequently used following the spread of various types of portable equipment. Because of its structure and chemical property, however, a secondary battery has the following disadvantages. In an overcharge state, gas due to solution vaporization may be generated and solution leakage may occur within a battery. In an overdischarge state, electrodes may be eluted within a solution. As a result, there is a possibility of the breakdown of the battery, the deterioration of battery characteristics and the like. Therefore, measures have been conventionally taken against overcharge and overdischarge in various manners.
FIG. 7 is an internal circuit diagram showing secondary battery packs 200 and 208 used for a battery such as a conventionally used lithium-ion secondary battery. In this case, the secondary battery packs are used as power supplies for a notebook-size personal computer (to be referred to as "notebook PC" hereinafter). The two battery packs 200 and 208 are connected to the internal circuit 205 of the notebook PC through back flow prevention diodes D1 and D2.
In the battery pack 200, a plurality of secondary battery cells (which are three secondary battery cells 202H, 202M and 202L in FIG. 7) are connected in series to thereby constitute a battery 202 as a whole. The negative voltage side of the battery 202 is connected to the negative terminal Batt- of the battery pack 200 and the source terminal of a discharge control P type field effect transistor (to be referred to as "FET" hereinafter) 203 is connected to the positive voltage side of the battery 202. Also, the drain terminal of the FET 203 is connected to the drain terminal of a charge control P type FET 204 and the source terminal of the FET 204 is connected to the positive terminal Batt+ of the battery pack 200. Battery voltage is thus outputted between the negative terminal Batt- and the positive terminal Batt+ of the battery pack 200 and then supplied to the internal circuit 205 of the notebook PC acting as a load. A charger detection terminal OCV is connected to the positive terminal Batt+ of the battery pack 200 to detect that a charger (not shown) is connected to the battery pack. In addition, a current limiting resistor R201 is inserted between the positive terminal Batt+ and the OCV terminal to prevent overcurrent at the time of erroneously connecting the battery pack 200 and the charger (not shown).
In the charge and discharge control circuit 201, a voltage detector circuit 220 is connected to each of the battery cells 202H, 202M and 202L and monitors the voltage value of each of the battery cells 202H, 202M and 202L. Each battery cell 202H, 202M or 202L has a structure independent of other cells due to the need to protect the cell against overcharge and overdischarge independently. If at least one of the battery cells 202H, 202M and 202L is turned into an overcharge state, then the output of the corresponding voltage detector circuit 220 is inverted and the gate terminal COUT of the charge control P type FET 204 is set at a high level by an overcharge control circuit 231 outputting a logic sum of the voltage detector circuits 220 to thereby make the FET 204 nonconductive and disconnect a charge path.
Conversely, if at least one of the battery cells 202H, 202M and 202L is turned into an overdischarge state, then the output of the corresponding voltage detector circuit 220 is inverted and the gate terminal DOUT of a discharge control P type FET 203 is set at a high level by an overdischarge control circuit 232 outputting the logical sum of the voltage detector circuits 220 to thereby make the FET 203 nonconductive and disconnect a discharge path. At the same time, this overdischarge state is latched by a latch circuit 240 and the charge and overdischarge control circuit 201 is turned into a standby state. At this moment, if the overcharge state is not detected, a bias circuit 250 is made inactive and the internal bias of the charge and discharge control circuit 201 is stopped to turn the circuit 201 into a complete dormant state. Then, no current is consumed in the charge and discharge control circuit 201, so that the secondary battery 202 which has supplied the internal bias is not consumed and the battery voltage is not decayed for a long time. Accordingly, it is unlikely that after the circuit 201 is halted in an overdischarge state, the battery voltage is unnecessarily decayed and battery characteristics unnecessarily deteriorates.
In recharge operation, the negative terminal and the positive terminal of the charger (not shown) are connected to the negative terminal Batt- and the positive terminal Batt+ of the battery pack, respectively, thereby turning the charger detection terminal OCV into a high level. Thus, the latch circuit 240 which has latched the overdischarge state is reset, the charge and discharge control circuit 201 is out of the standby state, the bias circuit 250 starts operation and the internal bias is applied. The gate terminal COUT of the charge control P type FET 204 is turned into a low level by the overcharge control circuit 231, the FET 204 is made conductive and thereby charge operation is started. This state continues until an overdischarge state is detected and the battery cells 202H, 202M and 202L are charged in this state until then.
Next, it is assumed that the battery pack 200 is in an overdischarge state and the other battery pack 208 is in a charge state. In this case, while the voltage of the positive terminal Batt+ of the battery pack 200 is low in level, the cathode of the diode D1, i.e., the cathode of the diode D2 is high voltage level and the diode D2 is reversely biased and a reverse leak current flows into the battery pack 200. If the voltage level of the charger detection terminal OCV is increased due to the leak current, there is fear that the charge and discharge control circuit 201 erroneously recognizes that the charger (not shown) is connected to the battery pack 200 and turns on the internal bias, and that charge operation is erroneously started. In this example, to prevent this erroneous operation, a leak current drawing N-type FET (M1) is connected between the charger detection terminal OCV and a reference voltage, and made conductive in a standby state to thereby let the leak current flow to the reference potential. By regulating the residual voltage of the charger detection terminal OCV to about 0.3V with respect to the leak current of about 300 .mu.A while the leak current drawing N-type FET (M1) is conductive, the charge operation by the latch reset is prevented from being erroneously started.
In the above-stated charge and discharge circuit 201, after the battery pack 200 is turned into an overdischarge state, the internal bias turns into a halt state under conditions that all of the battery cells 202H, 202M and 202L are not in an overcharge state, the consumption of the battery 202 is stopped and thereby the battery voltage is not decayed for a long time.
Nevertheless, there exists a leak current, though only slightly, in the battery 202 itself or the charge and discharge control circuit 202 or the like. Then, if the battery 202 is left untouched for a long time, the battery voltage gradually decays and the battery is completely discharged without any residual voltage in the end. If the battery is in such a state, no power is supplied to the charge and discharge control circuit 201 and the circuit 201 becomes inoperative to thereby make it disadvantageously impossible to carry out recharge operation.
FIG. 8 is a circuit diagram showing a battery pack 300 which is proposed to avoid the above-stated disadvantages. Specifically, a charge controlling output circuit Z in a charge and discharge control circuit 301 is provided to avoid the disadvantages. In the charge controlling output circuit Z, the gate terminal COUT of a charge control P type FET 204 is connected to the reference potential through a resistor R302 to maintain the potential of the gate terminal COUT of the charge control P type FET 204 at a low level irrespectively of battery voltage. Therefore, when a charger (not shown) is connected to the battery pack 300, the charge control P type FET 204 is made conductive irrespectively of battery voltage and a battery 202 can be recharged even after it is completely discharged.
Further, to turn the charge control P type FET 204 into a nonconductive state during overcharge, the charge controlling output circuit Z is provided with a PNP type bipolar transistor Q302 having a collector terminal and an emitter terminal connected to the gate terminal COUT of the charge control P type FET 204 and to a charger detection terminal OCV, respectively and having a base terminal to which the charger detection terminal OCV is connected through a resistor R301. Further, an NPN type bipolar transistor Q301 for base bias application is connected to the base terminal of the transistor Q302. The base terminal of the NPN type bipolar transistor Q301 is controlled by an overcharge control circuit 231. When the battery 300 is overcharged, the NPN type bipolar transistor Q301 is made conductive and applied with a bias current, the PNP type bipolar transistor Q302 is made conductive and the gate terminal COUT of the charge control P type FET 204 is connected to the charger detection terminal OCV. If a charger (not shown) is connected to the battery pack 300, the charger detection terminal OCV is high in level. Therefore, no potential difference exists between the gate terminal COUT and the source terminal of the charge control P type FET 204 and the charge control P type FET 204 becomes nonconductive, thereby making it possible to prevent erroneously charging the battery during overcharge.
In this example, however, the charge controlling output circuit z cannot disadvantageously operate normally if a reverse leak current flows into the battery pack 300 through a diode D2. That is, if at least one of the battery cells 202H, 202M and 202L is turned into an overdischarge state and the battery cell in an overcharge state is mixed with the battery cells in a state other than the overdischarge state, then a leak current drawing N-type FET (M1) is made conductive to prevent a latch circuit 240 from being reset to latch the overdischarge state and the charge controlling output circuit Z operates to connect the gate terminal COUT of the charge control P type FET 204 to the charger detection terminal OCV so as to stop charge operation. The potential of the charger detection terminal OCV at this moment is, however, decreased to about 0.3V by making the leak current drawing N-type FET (M1) conductive and the potential is lower than that of the positive terminal Batt+, i.e., that of the source terminal of the charge control P type FET 204 through a current limiting resistor R201. Owing to this, even if the lowered voltage of the charger detection terminal OCV is connected to the gate terminal COUT of the charge control P type FET 204 through the charge controlling output circuit Z, the potential difference between the gate terminal COUT and the source terminal of the charge control P type FET 204 remains higher than the threshold voltage of the FET 204 and the FET 204 cannot be turned into a nonconductive state depending on conditions. The charge controlling output circuit Z cannot, therefore, operate normally.
As stated above, the charge and discharge control circuit 201 shown in FIG. 7 is capable of reducing the consumption of the battery 202 after overdischarge as much as possible, stopping the unnecessary decrease of battery voltage to thereby prevent the deterioration of battery characteristics and preventing the battery from being erroneously charged by the other battery pack 208. The charge and discharge control circuit 201 has, however, disadvantage in that the battery cannot be recharged after the battery voltage is completely discharged.
The charge and discharge control circuit 301 shown in FIG. 8 allows the battery to be recharged after the battery voltage is completely discharged. However, the circuit 301 has disadvantage in that the circuit 301 cannot function in conformity with the function of preventing the battery from being erroneously charged by the other battery pack 208 and in that erroneous charge cannot be prohibited when a reverse leak current flows into the battery through the diode D2 from the other battery pack 208.